I want to be sure you are thinking this through, considering all angles, and making an informed, wise decision.

You can try to go after their renters insurance, but since you don't have proof, you're probably not going to get anywhere.

Find yourself a good team who builds ADU's has a project management team and of course has all the licenses and insurances to protect your property.Quote from @Michael Adamo: Hi BP Community,I’m exploring adding one or two ADUs to a property I own in the Miracle Mile area, near 3rd Street and La Brea in Los Angeles.

First and foremost, I'm in Minneapolis / Saint Paul, so I'd like to know if you have any suggestions for 1) what investor meetups I should attend, and 2) recommendations for building my team (CPA, real estate lawyer, handyman / contractor, insurance broker, and agent (I have one but they don't seem to have much investing experience)).

For insurance, I’d recommend working with a local broker to explore the best rates.

Quote from @Raymond Whitley: Not sure if Goosehead is in your area, but a broker like someone at this company can really help you in challenge times for insurance like this current market with so many carriers dropping out completely or raising rates on everyone. 

Personally speaking, from an investment broker and investor standpoint, I feel single family investing is the riskiest in some investment ways in comparison to larger investments due to the fact that it only takes 1 repair or tax/insurance hike to put you under or lose all income.

It sounds like a more personal decision based on your goals.

And STRs really help because you don't have the large dings that long term rentals have - i.e tenants damaging the property to where you have huge make ready costs (the str insurance covers that), no collections or evictions issues. 

You’ll want to make sure the rental income comfortably covers the new mortgage, taxes, insurance, and any maintenance costs.